Although research to develop a negative active material having a high capacity based on metallic materials such as Si, Sn, and Al has actively been undertaken, such research has not yet succeeded in applying metals to a negative active material. This is mainly due to problems with the deterioration of the cycle life characteristics as a result of alloying of the metallic materials such as Si, Sn, and Al with lithium during charge and discharge, because the alloy expands and contracts in volume, which produces excessively small micro-particles.
In order to attempt to solve these problems, an amorphous or micro-crystalline Si foil obtained from CVD and a sputtering procedure as a negative active material metal has been suggested in Japanese Patent Laid-Open Publication No. 2002-83594. The amorphous Si does not convert into micro-particles and gives improved cycle life characteristics during repeated charge and discharge, because the volume expansion of an alloy of the amorphous Si and lithium is smaller than that of a crystalline Si.
However, a larger capacity than the conventional graphite negative electrode requires a thick Si foil which requires a long formation time and high cost, and decreases conductivity, deteriorating battery performance.
Thus, the bulk crystalline Si is pulverized through mechanical pulverization under a high shear force, so that the crystalline lattice of Si is distorted to convert it into an amorphous state and to minimize an average diameter thereof, thereby obtaining amorphous Si powder.
The amorphous Si powder, however, has a wide diameter distribution between about several hundred nm to 1 μm, and the macro particles at about 1 μm deteriorate the cycle life characteristics because they are severely expanded and contracted which minimizes particle size during the charge and discharge.
It has also been attempted to use a mixture of mechanically pulverized Si powder and graphite. However, the mechanically pulverized Si powder has a wide diameter distribution, and it contains macro particles with a diameter of 1 μm in which volume expansion and shrinkage largely occurs, resulting in the deterioration of the negative electrode.